This invention relates to a method of manufacture of a depletion mode power MOSFET having a gate electrode formed of material that is refractory, or resistant, to high temperature encountered during thermal growth of oxide or any other high temperature encountered during device fabrication.
Material that is refractory, or resistant, to any high temperature encountered during device fabrication includes, for example, polysilicon; tungsten or other refractory metals; or titanium silicide or other refractory silicides formed from the known "polycide" process. For convenience, all such materials are referred to simply as "refractory".
Power metal oxide semiconductor field-effect transistors ("MOSFETs") are well known devices, and are typically used for power switching applications. As is known, a MOSFET includes source and drain regions of a first conductivity type, and a base region of a second conductivity type, which separates the source and drain regions. A conduction channel is formed in the base region to interconnect the source and drain regions and enable device conduction.
A depletion mode MOSFET uses a thin "depletion" channel region of the first conductivity type at the surface of the base region, which interconnects the source and drain regions. An electric field induced by a gate electrode above the depletion channel region empties such region of mobile carriers and thereby stops current conduction in the device. An enhancement mode MOSFET, on the other hand, does not utilize a channel layer of a first conductivity type beneath the gate electrode. Such device, instead, relies on an appropriate bias on a gate situated above the surface of the base region to induce an inversion channel in the base region, which conductively interconnects the source and drain regions.
The standard process for producing power MOSFETs is not applicable to depletion mode MOSFETs. In the standard manufacturing process, a highly-doped material, such as polysilicon, is patterned on the device to form a gate electrode. The gate electrode is then used as a mask to form a base region that is diffused at high temperature, such as 1175.degree. C., for an extended period of time, such as 120 minutes. If the gate electrode is used as the base region mask, the depletion channel region must be formed prior to forming the gate electrode. The high temperature thermal drive used to form the base region, however, causes the thin depletion region to grow in thickness and to be harder or impossible to deplete with typical gate voltages.
One approach to prevent the depletion region from growing in thickness is to use a metal gate electrode rather than a refractory gate electrode. This is because metal can be formed at a temperature that is relatively low and at which the depletion channel region does not grow in thickness. The resulting device, however, is complicated because two metal systems must be accommodated on the device upper surface: one for the gate electrode, and the other for the source electrode.
It would, therefore, be desirable to provide a method of manufacturing a depletion mode power MOSFET having a gate electrode formed of refractory material.
An additional drawback of conventionally-made MOSFETs is that their gate oxides are highly sensitive to ionizing radiation. Such radiation is known to induce charges in the gate oxide, which produces a shift in the gate-to-source threshold voltage. The gate-to-source threshold voltage decreases with increasing total radiation dose for N channel devices and increases with total dose for P channel devices. The gate drive circuitry must be designed to offset these threshold voltage shifts by overriding them with appropriate biasing levels. This complicates the control circuitry. A description of the shift in threshold voltage is described in more detail in a paper entitled "Radiation Resistance of Hexfets", contained at pages B-10 through B-12 of the Hexfet Databook of 1985, published by the International Rectifier Corporation, El Segundo, Calif.
It would thus be desirable to provide a power MOSFET having a gate-to-source threshold voltage that is close to constant regardless of exposure to radiation dosages up to 1 megarad, for example.